[Extensions] Don't allow content scripts on the New Tab Page

Extensions wishing to modify the New Tab Page should do so through the
use of the chrome url overrides [1] API, not through content scripts.
Disallow content script injection on the New Tab Page.

[1] https://developer.chrome.com/extensions/override

BUG=662610

Review-Url: https://codereview.chromium.org/2978953002
Cr-Commit-Position: refs/heads/master@{#487664}
7 files changed